This invention relates to diagnostic devices and, more particularly, to medical sensing devices used to detect energy in the audible range. The device of this invention is particularly useful for the detection of a broad range of frequencies of bio-acoustic signals or sound waves generated by the human body, particularly those by the heart.
Acoustical signals emanating by and from the human body are monitored and analyzed for a variety of purposes such as phono-cardiography. Phono-cardiography involves the detection of acoustical signals produced by the movement of heart muscle, heart valves and resultant blood flow which are transmitted through chest cavity tissues primarily to the chest and back skin surfaces. A sensor is typically placed at a predetermined, precordial location on the patient body surface to receive and transmit these signals.
In the past, a variety of sensors have been proposed or utilized to detect and analyze heart as well as other bio-acoustic signals. These sensor devices range from primarily mechanical devices, such as stethoscopes, to electronic devices, such as microphones and accelerometers. These prior art sensors have various limitations including the distortion and attenuation of sound waves, the necessity for continuous "hands-on" operator manipulation, and the inability to simultaneously detect high and low frequency signals.
A major problem with existing sensors is that they do not "couple" well to the body surface due to the impedance difference between the soft body tissues and the adjacent air. This impedance mismatch can result in acoustical energy losses exceeding 99 percent. Thus, only a small amount of an acoustical signal is transmitted from the body by present sensors or transducers.
Attempts in the prior art to deal with the coupling problem include disposing a hydrophone in a liquid medium, such as a gel or water. The hydrophone senses sounds in the liquid environment, as opposed to the gaseous fluid environment of a microphone. A problem with such a device, however, is the required use of a flexible membrane or diaphragm which permits the medium to closely contact the body surface. These and other prior art devices usually also require gel to be applied over the patient's skin for interfacing the membrane or diaphragm. This requirement is messy, time consuming and results in coupling losses between the gel and diaphragm boundary and between the diaphragm and transducer due to the interior fluid medium. Coupling losses of approximately 10 percent can result from sound wave transmittance across such flexible diaphragms. Further, hydrophonic fluids may chemically react with or degrade the diaphragm structure of the sensor devices.
Prior art sensors utilizing fluid mediums further have difficulty with suspended gas bubbles in the hydrophonic media which result in high sound wave attenuation or energy loss. These attenuation losses are primarily due to viscous forces and heat conduction losses associated with the compression and expansion of small gas bubbles caused by the traveling sound wave. The resultant gas bubbles in the transmission medium also cause sound wave scattering which further results in sound wave energy loss. The presence of gas bubbles affects the physical characteristics of the medium by altering its density and compressibility which effects sound wave speed and which can result in considerable acoustic energy reflection and refraction losses.
Despite the need for a bio-acoustic sensing device, particularly which overcomes the limitations of the prior art in the medical diagnostic art, and which provides reliable reception of sound waves, particularly in the sub-kilohertz (KHz) range, none insofar as is known has been proposed or developed.
Accordingly, it is an object of the present invention to overcome these prior art limitations and to provide a bio-acoustic signal sensing device that is effective and economical to make and use, that detects a broad range of heart sound frequencies, particularly low frequency sounds, and that minimizes heart sound wave distortion and attenuation.